Aerogels are a unique class of ultrafine cell size, low density, open-cell foams. Aerogels have continuous porosity and a microstructure composed of interconnected colloidal-like particles or polymeric chains with characteristic diameters of 100 angstrom. The microstructure of aerogels is response for their unusual acoustic, mechanical, optical and thermal properties. The mircrostructure imparts high surface areas to aerogels, for example, from about 350 m2/g to about 1000 m2/g. The ultra fine cell/pore size minimizes light scattering in the visible spectrum, and thus they can be prepared as transparent, porous solids. Further, the high porosity of aerogels makes them excellent insulators.
Traditional aerogels are inorganic (for example, silica, alumina or zirconia aerogels), made via the hydrolysis and condensation of metal alkoxides, for example, tetramethoxy silane.
Recently, organic aerogels from the sol-gel polymerization of resorcinol (1,3 dihydroxy benzene) with formaldehyde under alkaline condition have been developed as disclosed in U.S. Pat. No. 4,873,218 issued Oct. 10, 1989, and U.S. Pat. No.4, 997, 804 issued Mar. 5, 1991. Also, organic aerogels that are transparent and essentially colorless have been prepared from the aqueous, sol-gel polymerization of melamine with formaldehyde, see U.S. Pat. No. 5,086,085 issued Feb. 4, 1992. In addition, organic aerogels have been produced using precursor materials which include polyacrylonitrile (PAN), polymethylacrylonitrile (PMAN), cathechol/formaldehyde, phenol/formaldehyde, etc., or mixtures thereof, see U.S. Pat. No. 5,476,878 issued Dec. 19, 1995, U.S. Pat. No. 5,556,892, issued Sep. 17, 1006, No. 5,731,360 issued Mar. 24, 1998, U.S. Pat. No. 5,744,510 issued Apr. 28, 1998, and U.S. Pat. No. 5, 932, 185 issued Aug. 3, 1999. All the above-referenced US patents are hereby incorporated by reference.
Typically, these organic aerogels are formed by a sol-gel reaction of a precursor material solution in water, to which is added a base catalyst to initiate the reaction. The resultant gel is then dried, either by repetitive solvent exchange, or by supercritical extraction. If the gel can be dried in such a fashion that capillary forces will not collapse the structure, an aerogel is the result, and if not a xerogel is formed.